High strength aluminum coated steel



United States Patent 3,118,223 HEGH STRENGTH ALUMEJUM OUATED STEELWilhelm Schiill, Cologne-Dellbruck, and Clemens Eisenhnth,Cologne-Holweide, Germany, assignors to Amer can Chain & Cable Company,Inc, a corporation of New York No Drawing. Filed Mar. 29, 1960, Ser. No.18,226 Claims priority, application Germany Apr. 9, 1959 Claims. (Cl.29-528) This invention relates to aluminum coated steel, and isparticularly directed to the provision of an improved method forproducing aluminum coated steel or" high tensile strength. It is aparticular object of the invention to produce high strength aluminumcoated steel wire by a process which involves dipping the wire (or othershape) in molten aluminum to form the aluminum coating thereon.Satisfactory processes and equipment that can be used for hot dipaluminizing of wire have only recently been developed. Good coatings areformed by these processes, but with the limitation that the strength ofthe steel wire does not exceed about 170,000 lbs/sq. in. The productionof aluminized steel wire of greater strengths is not possible by theprocesses heretofore proposed, for during aluminizing the strength ofthe Wire is considerably reduced because of the high melting point ofthe aluminum bath. it has been attempted to counteract this wirestrength loss by a subsequent drawing operation. This, however, has beenonly partly successful, as the aluminized steel wire is not structurallyadapted to the extensive cold working necessary for developing a reallyhigh tensile strength (e.g., above 175,000 lbs/sq. in.). Thus, it hasnot been possible, by the processes presently known, to producealuminized steel wire of greater strength than 170,000 lbs/sq. in.

Efforts also have been made to lessen the loss of strength accompanyingthe aluminizing of steel wire by keeping the temperature of the coatingbath as low as possible through the addition of suitable metals, butimportant improvements have not yet been obtained by this approach.

This invention provides for the production or aluminized steel wire ofhigh strength by passing the cleaned wire through a molten aluminumcoating bath and drawing it in the same process. The invention departs,however, from procedures heretofore used in that the temperature of thebath is raised to above the values heretofore employed and is controlledin accordance with the carbon content of the wire passing through, sothat the utmost possible alteration in the wire to an austeniticstructure results.

Thus, in carrying out the new method, the bath of molten aluminum isheated to above the Ac point of the wire or other steel shape to becoated. The wire (or other shape) then is immersed in such bath, and isretained therein long enough to be heated to above its AC3 temperature.Thus, upon withdrawal of the steel wire or other shape from the moltenaluminum, it is in the substantially fully austenitic condition. It iscooled in air substantially to room temperature and thereafter severelyworked to develop the desired high tensile strength (above 175,000lbs/sq. in.).

in order to avoid the necessity of supplying heat to the bath to heatthe steel to above its Ac point, the wire or other shape may bepreheated in a protective gas atmosphere to a temperature near or aboveits Ac point. The resulting hot wire then is introduced into the moltenaluminum bath. Preheating the wire not only reduces the need forsupplying heat to the molten aluminum, but also reduces the time thewire need be held in the molten aluminum bath to be heated to above itsAc transformation point. Indeed, in some cases it is practical anddesirable to preheat the wire to above its Ac point and to introduce itat such temperature into the heated molten aluminum. All the heatrequired to maintain the aluminum bath at the desired temperature may insome cases be supplied in this fashion. Even in such case, however, themolten aluminum must be heated to above the AC3 point of the wire toavoid quenching and hardening it.

The actual temperature to which the molten aluminum must be heateddepends of course on the carbon content of the steel wire, for the Acpoint depends on carbon content. For example, a steel wire containing0.5% carbon has an Ac point of about 1435 F.; and a steel wirecontaining about 0.85% carbon has an Ac point of about 1365 F. Thealuminum bath is, in accordance with the invention, heated to above suchtemperature, depending on the particular steel being aluminum coated.

Wh n the wire is preheated to near or above its Ac point, it should beprotected by a non-oxidizing atmosphere to prevent substantial ox dationof its surface prior to its introduction into the molten aluminum. Thealuminum coating which adheres to the wire as it is withdrawn from thebath protects it from oxidation, and hence the wire can be allowed tocool in air without suffering any undesirable chemical attacl By meansof the method of this invention it is possible to produce aluminumcoated high carbon steel wires of very high tensile strength. Forexample, aluminum coated steel wires containing 0.5 carbon havingtensile strengths exceeding 200,000 lbs/sq. in., and similar wirescontaining 0.85% carbon having tensile strengths substantially exceeding250,000 lbs/sq. in, can readily be produced. Steel wires or" suchstrength, and possessing a high degree of hardness, are eminentlysuitable for use as spring wire, in high strength bridge cables and wireropes, and for other purposes requ ring steel of very high tensilestrength. Moreover, these high strength products possess the notableresistan e to corrosion that characterizes aluminum coated articles.

Following is an example of the method of this invention. A steel wirecontaining 0.85% carbon, 0.102 inch in diameter, was heated in aconventional patenting furnace to a temperature of about 1550 F. Theheated wire was rapidly transferred to a bath of molten aluminummaintained at a temperature of about 1370 F. The wire was then withdrawnfrom the aluminum bath with an adherent aluminum coating and was cooledin air to room temperature. Thereafter, it was cold drawn to a finaldiameter of 0.039 inch. The drawn wire had a tensile strength of 265,000lbs/sq. in. and withstood 30 full turns when twisted about its axis overa length of times its diameter.

We claim:

1. The method of producing high strength aluminum coated steel whichcomprises heating a bath of molten aluminum to a temperature above theAc point of the steel, immersing the steel in such bath to form analuminum coating thereon, Withdrawing the steel from the bath in thesubstantially fully austenitic condition, cooling the steelsubstantially to room temperature, and severe ly working the cooledaluminum-coated steel to develop therein a high tensde strength.

2. The method of producing high strength aluminum coated steel Wirewhich comprises heating a bath of molten aluminum to a temperature abovethe Ac point of the wire, immersing the wire in such bath to form analuminum coating thereon, withdrawing the wire from the bath in thesubstantially fully austenitic condition, cooling the wire substantiallyto room temperature, and severely working the cooled aluminum coatedwire to develop therein a high tensile strength.

3. The method of producing high strength aluminum coated steel Wirewhich comprises heating a bath of molten 52 aluminum to a temperatureabove the Ac point of the Wire, preheating the Wire to a temperaturenear its AC3 point, immersing the preheated wire in the molten aluminumand withdrawing it therefrom only after it has attained a temperatureabove its A0 point and is in substantially the fully austeniticcondition, cooling the wire substantially to room temperature, andseverely Working the cooled aluminum coated wire to develop therein ahigh tensile strength.

4. The method of producing iigh strength aluminum coated steel wireWhich comprises heating a bath of molten aluminum to a temperature abovethe A0 point of the wire, preheating the Wire in a protective gasatmosphere to a temperature near its A0 point, immersing the preheatedWire in the molten aluminum and maintaining it therein until it isheated to above its Ac point, withdrawing the wire from the bath in thesubstantially fully austenitic condition, cooling the wire substantiallyto room temperature, and severely Working the cooled aluminum coatedwire to develop therein a high tensile strength.

5. The method of producing high strength aluminum coated steel wirewhich comprises heating a bath of molten aluminum to a temperature abovethe Ac point of the wire, preheating the Wire in a protective gasatmosphere to a temperature above its AC3 point, passing the thusheatedWire through the bath of molten aluminum and withdrawing it therefrom inthe substantially fully austenitic condition, cooling the resultingaluminum coated wire in air substantially to room temperature, andthereafter severely working the wire to develop therein a high tensilestrength.

References Cited in the tile of this patent UNITED STATES PATENTS1,409,017 Ortiz Mar. 7, 1922 2,082,622 Fink June 1, 1937 2,191,598Swartz et al. Feb. 27, 1940 2,455,457 Whitfield et al. Dec 7, 19482,543,936 Reynolds Mar. 6, 1951 2,686,355 Lundin Aug. 17, 1954 2,785,084Lundin Mar. 12, 1957 3,057,050 Hodge et a1. Oct. 9, 1962

1. THE METHOD OF PRODUCING HIGH STRENGTH ALUMINUM COATED STEEL WHICHCOMPRISES HEATING A BATH OF MOLTEN ALUMINUM TO A TEMPERATURE ABOVE THEAC3 POINT OF THE STEEL, IMMERSING THE STEEL IN SUCH BATH TO FORM ANALUMINUM COATING THEREON, WITHDRAWING THE STEEL FROM THE BATH IN THESUBSTANTIALLY FULLY AUSTENITIC CONDITION, COOLING THE STEELSUBSTANTIALLY TO ROOM TEMPERATURE, AND SEVERELY WORKING THE COOLEDALUMINUM-COATED STEEL TO DEVELOP THEREIN A HIGH TENSILE STRENGTH.